As devices such as personal computers, video cameras, and cell phones are downsized in the fields of information-related devices and communication devices, lithium secondary batteries have been practically and widely used as power sources in these devices because of high energy density. Meanwhile, in the field of automobiles, there is demands for urgent development of electric vehicles, because of environmental and resources problems, and lithium secondary batteries have also been examined as power sources for such electric vehicles.
The lithium secondary battery uses chemically highly active lithium, a highly combustible electrolyte, and a cathode active oxide material poor in thermal stability in a charged state, and thus the battery should be handled with great care. Particularly, one of main advantages of the lithium secondary battery is the ability of the battery to be repeatedly discharged and charged, thus requiring intensive countermeasures against improper use during charging, specifically against overcharging. It is known that generally when a lithium secondary battery remains overcharged, a decomposed gas of an electrolyte is generated once the potential of a cathode exceeds the decomposition potential of the electrolyte, and as the gas is generated, the internal pressure of the battery is increased. Accordingly, there is demands for a lithium secondary battery that can detect an overcharged state instantly.
Patent Document 1 discloses a nonaqueous-electrolyte secondary battery wherein a conductivity regulating material with electric conductivity varying significantly by doping/dedoping is added to a cathode active material having a phosphate compound of olivine structure. This secondary battery, when overcharged, interrupts current by a drop in the electric conductivity of the conductivity regulating material Patent Document 2, on the one hand, discloses a battery having a layer of an electroconductive polymer formed between a cathode layer and a cathode current collector layer. In this battery, the electroconductive polymer upon malfunction such as overcurrent or overvoltage has increased resistance with which the battery circuit is interrupted. However, these batteries mentioned above use an electroconductive polymer etc., so there are problems such as an increase in costs and in the number of steps for production.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2002-216770
Patent Document 2: JP-A No. 10-199505